Gas anchor



OIL OUT FIG. 2

FIGB

E. V. WATTS GAS ANCHOR OWLY` F|G.l

,Filed April 3, 1945 ROTH LAYER MOVING RAPIDLY PRO H :NvENToR A'rToRNEYV. WATTS Patented Apr. 9, i946 NITED sTTs ENT OFFICE GAS NCHR Euclid V.Watts, South Pasadena, Calif., assignor to Socony-Vacuum i] Company,Incorporated, N ew York, N. Y., a corporation of New York ApplicationApril 3, 1945, Serial No. 586,288 1 claim. oL 10aa-203) solution in theoil by. decrease in pressure or, l0

occasionally, by turbulent ow of oil and gas through casing perforationsor vother restrictions.

In the case of a newly tapped reservoir this froth formation may bebeneficial in Vreducing the hydraulic head of the column of oil in thel5 tubing, thus causing a well to flow naturally which would have to bepumped if the froth were excluded and the tubing iilled with liquid`oil. But when pumping is required, the presence of the froth in the pumpbarrel materially reduces pumping eiiciency. Not only does the froth voccupy uselessly a part of the displacement volume of the pump barrel, butif in suiiicient quantity, it may expand and contract with the plungerstroke to such extent as to causegas-lock and 275 total cessation ofliquid delivery.

It is therefore customary, in cases in which froth tends to form at thepumping level, toprovide the pump with a so-called gas anchor.

This device, in its simplest and most common form, is little more thanan elongated cup, open at its upper end except for an attachment tothepump, and provided with an4 inner y tube (the skeeter bill) whichextends from the pump intake to a point adjacent the lower end of thecup.

This device diverts away from the pump suction any large gas bubblesrising through the pool of oil in the bottom of the hole routside the Aanchor. It also, in theory, permits gas bubbles inducted into or evolvedwithin the cup toW rise Y and escape through the open upper end while asolid (in the sense of unbroken) annular column of oil passes down tothe end ofthe skeeter bill and thus into the pump intake,

This theory is borne out in practice only in instances in which theupward velocity of gas bubbles relative to the oil exceeds the downwardvelocity of the annular column within the anchor and in which the iilmproperties of the liq- 5o uid permit the rupture of bubbles as fast asthey accumulate at the top of the anchor, thereby completing theseparation of the gas phase from the liquid phase.

fa more or less xed quantity, being determined by the quantity of oilflowing into the pump intake and the cross-sectional area of the largestcolumn which can be accommodated within the casing. The upward velocityis a function of theI viscosity of the oil and also of the size of thebubble, increasing (in an oil of given viscosity) as the square of thediameter of the bubble.

it follows, and is well recognized, that while these simple anchorsfunction to advantage in wells giving small yields of light,lowviscositycrude, their eiliciency falls off very rapidly with 'increasing yield,enhanced velocity and reduced bubble dimensions. For these reasons theyare of little if any utility in excluding gas from pumps handling theheavier class of crudes, or large volumes of any viscosity.

The structure described herein is an improvement over the conventionalform of gas anchor in two major respects: iirst in materially increas-`ing the time required for the oil column iiowing downwardly within theanchor to travel any given vertical distance; second in reducing thevertical distance through which the gas bubble must rise through the oilbody before escaping from it.

The simple structure of theinvention and the manner in which itfunctionsare illustrated in the attached drawing, in which Fig. 1 is asection through a casing and helical gas anchor, showing the pump intaketube in elevation;

Fig. 2 is a diagram used to illustrate the functions oi the structure ofFig. 1, and l y Fig. 3 is a section through a modied structure whichutilizes the same' operating principle as that of the rst gure.

Referring first to Fig. 1, I0 is a well casing having perforations l l-II through which oil enters, the oil carrying entrained gas. An anchorshell l2 surrounds the pump suction extension tube I3 and contains asheet metal helix I4 closely ntting both thetube and the shell. Theanchor is suspended from the suction tube by a disc I5 and the upper endof the tube screws into the suction end of a deep well pump not shown. Aperforation l6 allows oil to pass from the exteriorfto the interior ofthe anchor shell and one or more perforations il permit the escape ofgas and froth from the upper end of the anchor tube into the casing.d A

The opening I should be located more or less midway the length of theanchor tube l2 and should be in the i'orm of a deep slot situatedslightly above the helix-that is to say,the lower The downward velocityis, in any given case, edge of the slot shouldv lie far enough above theillustrated diagrammatically in Fig. 2, which represents an extensionofthe entire lengthof the helical channel I9 included between the turnsof the worm.

Oil with entrained gas enters the channel at I6 and flows downwardlyover the upper face 20 of the worm, finally reaching .the lpool I8 fromwhich oil is drawn by the pump. While so iiowing, gas in the forms offree bubbles and of froth, rises to the surface and the liquid ,volumeof the stream diminishes. 'The free gas bubbles rise through the frothlayer until they encounter the lower face 2l of the worm, which theyfollow upwardly to the point of escape l1, either as large individualbubbles or as a rapidly moving thin stream.

The froth which rises to the surface of the oil stream consists of thesmaller gas bubbles separated by av greater or lesser thickness of oil.So-

long as the proportion of oil in the mass'is sufficient to permit thebubbles to retain .their globular form, the froth has approximately thesame fluidity as the oil, but as the oil drains out from between theglobules they become closer together and the fluency diminishes. The nalstage in oil drainage is that at which the bubbles are separated only byextremely thin oil lms and the bubbles are distorted by mutual contact.

At Athis, stage the froth will not flow of its own volition, but isforced upwardly between the turns of the worm by the superior weight ofthe enteringy oil, occupying whatever space is left between theupflowing gas layer and the'downow, ing oil stream.

During this upward movement, which carries the froth past the oil inletand toward or to the upper en'd of the helical channel, the froth issubjected to forces which tend to rupture the oil films, permitting thegas to escape and the oil to coalesce into globules. 'Ihese forcesconsist principally of the internal movements occasioned by relativelyrapid movement of the interior of the helical column and retardation ofthe faces in contact with the walls, and of the agitation of the frothmass by largegas bubbles forcing their way over its upper surface. Thisfroth resolving action continues throughout the length of the helicalchannel and under favorable circumstances may break down the froth asfast as it is separated from the entering oil. In that event only gasescapes from the upper end of the channel while the small amount of oilseparated from the froth returns along the bottom of the channel torejoin the entering stream. Ordinarily, resolution will be less thanperfect and some very light froth, containing only a minute amount ofoil, will be returned to the space between the casing and the anchorshell, but at a level materially above the oil inlet.

'The advantage of this structure over the con,

ventional form (which would be represented by removing the worm. fromthe form of Fig. 1 and admitting oil at the upper end of the anchorshell)l will be evident from the above description. In the prior artform the rising gas has to traverse the entire length of the downiiowingoil column before escaping. The critical velocity of oil flow is thatwhich equals the rate of rise of the smallest gas bubbles which must beremoved in order to bring the oil into condition to be pumped, and thiscritical velocity is not changed by lengthening the distance between theinlet point and the lower end of the shell. In the helical form the gasseparates from the oil at an obtuse angle and must rise through an oillayer of less depth than the vertical distance between adjacent turns ofthe worm in order to escape from the oil flow into the froth mass,moving in the opposite direction and fromwhich it cannot return. Thus inthe helical form the critical limitation is not the velocity of the oilstream, but the time of residence of the oil in the channel, and thistime may be increased by lengthening the anchor up to such limits as arefixed by the desired location of the pump itself.

The most desirable leyel for oil inlet I6 will vary with the conditionof the oil column in the casing and with the rapidity with which the gas'and froth separate from the entering oil. The

portion of the anchor below the oil inlet performs the stripping of theoil, and the length of this portion must be suiiicient to effect asubstantially clean separation before the oil enters the pump suction.On the other hand, the portion of the channel above the inlet performsthe greater part of the froth resolution and thus the longer thisportion of the channel the less will be the quantity of frth returned tothe casing outside the anchor. Also, the greater the distance betweenthe froth outlet and the oil inlet, the smaller will be the chance offroth circulating back to l the inlet with the entering oil.

`Generally speaking, oils of high viscosity or in y which the gas isvery finely dispersed will require that the Oil inlet be at a relativelyhigh level, while with less viscous oils or those in which the gas iscoarselyr dispersed it may with advantage be placed lower. The optimumposition for the inlet in any given well can be determined only byexperiment with shells having inlets at different levels. y

A modified form of the device, using a different structure but the samefunctional principles, is shown in Fig.l 3. In this form the worin ofFig. l

, is replaced by a plurality of plates 23 dividing the annular spacebetween shell I2 and pump tube i3 into chambers. These chamberscommunicate through downwardly directed tubes 2@ and upwardly, directedtubes 25, each of which should extend at least half way through thedepth of the adjacent chamber. The inlet i6 for oil and gas may beplaced immediately below the outlet l1 for gas and froth, as in Fig. 3,or at a lower level as in Fig. 1. The two communication tubes 2Q and 25'should be on the same side of the plate in which they are located, withthe pairs of tubes alternating from side to side in descending fromplate to plate.

Oil and entrained gas enter the inlet opening l and iiow across theplate to the downpipe 24,

passing thus to the lower portion of the chamber te lighter gas andfroth ow over the liquid surface,

aaoasao in a contrary direction. and into the upper part of the chambernext above. i

'I'he gravity diilerence between the oil owing downwardly and 4the gasand froth being displaced upwardly is the soie' motivating forceavailable to produce now and care must be taken not to make thecommunicating vpipes too narrow. This form of the device is highlyeective in both stripping and breaking froth but has less capacity forany given form.

As the pressure drops across theI partitioning members in either or theforms described are. wholly negligible, either the worm oi' Fig. 1 orthe plates and tubes of- Fis. 3 may be of light sheet metal so long asit is capable of resisting corrosion by saline well fluids.

I claim as my invention:

In a gasanchor: an inner tubular member: a

set of dimensions than the helical substantially closed shell and spacedfrom said tubular member to define a vertical ann nular spacecommunicating with the interior of said tubular member at its lower end;partitions,

atleast approximately horizontal, dividing said space into a successionof chambers; means af-Y fording communication between the bottom of eachchamberiand the lower part of the chamber -next below; means affordingcommunication between the top of each chamber and Vthe upper part of thechamber next above; means for ad- Y mitting a stream of gas-containingoil intothe from said oil stream andflowing countercurrent thereto andthereove'r. A

EUCLID V. WATTS.

